Please visit this Solr wiki page for more information: https://cwiki.apache.org/confluence/display/solr/UpdateXmlMessages

You can use XML requests to:

• Delete documents by id: deletes a document that match the specified id.
  
  
• Delete documents by query: deletes documents that match the specified query.
  
  

Here's the structure of the delete operation:
<delete> <id> <query>

Here's a sample XML message to delete an existing document by its id:
<delete> <id>1</id> </delete>

Here's a sample XML message to delete existing documents by query:
<delete> <query>id:1</query> </delete>
To delete all documents:
<delete> <query>*:*</query> </delete>

Here's an example of an http request to delete a document by id:
$ curl http://localhost:8983/solr/COLLECTION-NAME/update?commit=true \ -H "Content-Type: text/xml" \ --data-binary '<delete><id>1</id></delete>'
Here's an example of an http request to delete documents by query:
$ curl http://localhost:8983/solr/COLLECTION-NAME/update?commit=true \ -H "Content-Type: text/xml" \ --data-binary '<delete><query>*:*</query></delete>'

Here's an example of an http request that uses the parameter stream.body:
http://localhost:8983/solr/COLLECTION-NAME/update?stream.body=<delete><query>*:*</query></delete>
In order for this request to work, you need to enable Streaming in "solrconfig.xml":
<requestParsers enableStreamBody="true" />
If you want to update the "solrconfig.xml" file directly in the Configset, make sure to reload all collections that use the updated Configset.
Please visit this page for more details: http://lucene.apache.org/solr/guide/8_5/requestdispatcher-in-solrconfig.html